There is a theorem by Hawking that the topology of black hole (roughly, the shape of the event horizon) is spherical. This is assuming no cosmological constant, and the metric is asymptotically flat (Minkowski). If the metric is asymptotically de Sitter (here you have positive cosmological constant), the result also holds. However in asymptotically anti-de Sitter case (negative cosmological constant), then it is possible to have non-trivial (non-spherical) event horizon - e.g. a toroidal black hole. Such black holes are called topological black holes.